Id appreciate an opinion from an experienced user of ArcGIS and its Hydrologic Functions.In order to develop an analysis protocol to my research project on roads drainage in forested areas, I created a mask with a very small portion of the area I'm studying, between two arms of a lake (about 300 km2), with the DEM I have to the whole. I tried to run FLOW DIRECTION, SINK, FILL and then the FLOW ACCUMULATION, as indicated in the help file ("Deriving runoff characteristics"). In a first try, based on the functions sintaxe, I used the output of each function as the input of the following. I noticed that the result of the FILL was quite the same layer/attribute table of its input, the result of SINK; and the final result, from FLOW ACCUMULATION, was a layer with no attribute table! (showed in light grey when right clicking the layer). Then I reinterpretated the help flowchart and decided to run the FILL using the exit from FLOW DIRECTION directly as its input (the same I had used as an input to the the SINK, which showed a high number of sinks). The result was more consistent, a layer visually quite similar to the input one (at the 1:100,000), but with different values to the 255 registers. HOWEVER, using this raster as an input to FLOW ACCUMULATION led to an apparently infinity loop! It has been runing for 6 hours, when it took me only 1 hour to do the whole process (with many further steps) manually, over the 1:100,000 paper chart. The help file actually prevent us that this may happen, but only if we use an input not created with FLOW DIRECTION. My input is a depressionless DEM created with FLOW DIRECTION, followed by the FILL function. If anyone has time and patience to read me, could you be kind enough to let me know your best guess why I'm not successful in defining the stream network? Moreover, would I need ArcHydro and/or HEC-GeoHMS to do that simple analysis of a big area, with thousands of culverts points (plus the delineation and calculation of main attributes of the watersheds at each culvert). Thank you very much for you attention!

I think you are not following the proper order of the steps. Doing it they way you are explaining does not make sense to me.

It helps me to think of the DEM as a physical model I need to manipulate a little before the water will shed off it "correctly". Think of the grids in he DEM as tiles with different elevations. The DEM usually includes grids that are not a the flowline of the creek (bridges/culverst) or poorly defined watersheds such as areas in the flat lands. So, I have to lower the grid in some places and raise it in others in order to make the water flow where is should. Then there are all the minor imperfections in the DEM of small local sinks that need to be filled so the water will flow in the right direction.

Once I'm done filling the sinks, I want to have a "hydrologically correct" DEM. Once I have a good filled DEM, the rest should be correct also. With the filled DEM (fil), the flow direction will be "correct" and then the flow accumulation will also be correct. If I find later that an important stream is in the wrong place or an important watershed boundary is incorrect, then I have to go back and correct the fil DEM.

If you have Arc Hydro installed, the order of steps in the Terrain Processing in generally the same order as the functions are listed in the Terrain processing menus. You probably don't need to use all of them.

My steps using Arc Hydro are as follows: (You can follow these in general with in the Spatial Analyst Tool>Hydrology, but there are not parallel tools there to all the Arc Hydro tools.)

1) Terrain Processing>DEM Manipulation>DEM Reconditioning. Use a polyline layer for the streams to "burn" in the streams where they should be. This lowers the DEM in the vicinity of the stream features. Many times the DEM will not provide enough definition for the subsequent processes to define streams. It can be helpful to put a short line just a bridges or culverts to "breach" them and avoid fill behind them and flow directions around them. You may need to put in polylines for the streams and major drainage systems in some places.2) Terrain Processing>DEM Manipulation>Build Walls. Only do this if you need to define a watershed "ridge" in a flat area of the DEM.3) Terrain Processing>DEM Manipulation>Fill Sinks4) Terrain Processing>Flow Direction5) Terrain Processing>Flow Accumulation6) Terrain Processing>Stream Definition7) Terrain Processing>Stream Segmentation8) Terrain Processing>Catchment Grid Delineation9) Terrain Processing>Catchment Polygon Processing10) Terrain Processing>Drainage Line Processing11) Terrain Processing>Adjoint Catchment Processing12) Terrain Processing>Drainage Point Processing

The above steps assume a simple dendritic system. If you compare the steps to the Arc Hydro menus you will see that there are a lot of functions that I don't typically use.

A very clear and detailed explanation, it will certainly help! I'm going to install HEC-GeoHMS (with ArcHydro 2.0), following your tips in another thred I had posted in ArcHydro forum, taking into account Stuart's remarks about the software, before continuing to work in my pilot area. I come back to the forum later, if I still have problems (once I'm new in GIS work). I appreciate your help!

I don't know if you have noticed, but my problems/post was from early 2012. Those and other Mark Boucher and Stuart answers helped me a lot solving the problems (although it wasn't even necessary for me to use HEC in my small pilot area, of 10,000 km2). Unfortunatelly, I finished my work many months ago and I can't remember details of how I succeed to tell you. Besides, my ArcGIS sudent's licence expires in 3 days and I don't intend to renew it, once I'm not working on that project anylonger. But the forum helps a lot! Don't panic and before you post your problems, make an exhaustive check of the detailed instructions posted in the specialized fora by specialists like them and you'll succeed too. Good luck!

My first advice is to not run this with the data on a network. Put it on your local PC. It speeds things up dramatically and avoids many unexplainable errors. You can check out the thread linked below my signature. Many of the common Arc Hydro errors and issues are posted there.